Swamp sparrows are found across North America in a variety of wetland habitats. Unlike most swamp sparrows, the coastal plain subspecies is found in tidal, brackish wetlands and needs to adapt to a high salinity environment.
Coastal plain swamp sparrows differ from all other breeding populations of swamp sparrows:
Zoom in to the images below to view the differences. The top image is of a coastal plain swamp sparrow, the bottom image from an inland bird.
It is not known why sparrows that inhabit salt marshes should have larger bills, but it may be that they forage extensively in the tidal mud for food. The darker and less rusty plumage probably helps to hide the bird from predators as it forages on gray mud.
Mud in tidal marshes tends to be grayish because of the presence of iron sulfide. Freshwater mud tends to be browner ("rust-colored") because of the presence of iron oxide.
The darkening of the plumage is not a result of something in the birds' diet or environment, baby birds of both subspecies that were hand-reared in captivity and fed identical diets molted into the plumage of their parents.
The selection pressure on swamp sparrows in tidal marshes to be darker, grayer, and to have larger bills must be intense, DNA analysis indicates that Swamp sparrows only recently (10 - 15,000 years ago) colonized tidal marshes.
The grayer plumage is consistent with other sparrows that have similar species or subspecies that live in salt marshes and inland, for example song sparrows and sharp-tailed sparrows.
In addition, many other kinds of vertebrates (e.g. shrews, voles, rails, Marsh wrens, ...) also have darker or grayer forms that live in salt marshes.